QUATERNARY FAULTS IN NORTHWESTERN BIG BEND NATIONAL PARK, SOUTHERN TEXAS

Pleistocene pediment surfaces in the northwestern part of Big Bend National Park (in Bone Spring and Persimmon Gap quadrangles) are offset about 15 m by a 10 km-long, northeast-dipping normal fault. Few other Quaternary faults have been reported around Big Bend Park. The offset pediment surfaces are mostly cut on sub-horizontal, weakly consolidated sandstone and mudstone of the Cretaceous Aguja Formation that is intruded by Tertiary basaltic sills. This bedrock is overlain by about 2-3 m of pebble to cobble-size pediment gravel with a sandy to silty matrix. A 50-cm-thick, stage III+ petrocalcic horizon preserved in the upper part of the gravel suggests a middle Pleistocene age for the pediment surface. At one location along the fault, basalt in the footwall is juxtaposed against eroded pediment gravel in the hangingwall. The fault plane in bedrock strikes N. 60° W. and dips 70° NE. and slickenlines indicate that movement was mostly dip-slip. The northwest fault trend is subparallel with other major normal faults in the northern part of the park that formed perpendicular to northeast-southwest directed basin-and-range extension. The fault was discovered during fieldwork to update the geologic map of Big Bend National Park for the National Park Service.